This application relates to optical filters based on optical resonators and cavities.
Optical filters have a wide range of applications. One type of commonly used optical filters is optical bandpass filters where optical spectral components within a spectral window transmit through the filter while other spectral components outside the spectral window are rejected. Optical resonators such as Fabry-Perot resonators may be used as such bandpass filters.
An optical whispering-gallery-mode (“WGM”) resonator is a special optical resonator and supports a special set of resonator modes known as whispering gallery (“WG”) modes. These WG modes represent optical fields confined in an interior region close to the surface of the resonator due to the total internal reflection at the boundary. Microspheres with diameters from few tens of microns to several hundreds of microns have been used to form compact optical WGM resonators. Such spherical resonators include at least a portion of the sphere that comprises the sphere's equator. The resonator dimension is generally much larger than the wavelength of light so that the optical loss due to the finite curvature of the resonators is small. As a result, a high quality factor, Q, may be achieved in such resonators. Some microspheres with sub-millimeter dimensions have been demonstrated to exhibit very high quality factors for light waves, e.g., ranging from 103 to 109 for quartz microspheres. Hence, optical energy, once coupled into a whispering gallery mode, can circulate within the WGM resonator with a long photon life time. Such hi-Q WGM resonators may be used in many optical applications, including optical filtering.